The present invention relates generally to magnetic resonance imaging (MRI), and more particularly to a system and method for water and fat separation in MRI pulse sequences employing steady-state free precession.
MRI pulse sequences that take advantage of steady-state free precession (SSFP), e.g. Siemens"" xe2x80x9cTrue-FISPxe2x80x9d, General Electric""s xe2x80x9cFIESTAxe2x80x9d, and Phillip""s xe2x80x9cBalanced FFExe2x80x9d, allow fast imaging without excessive saturation of signals. In practice, SSFP is extremely useful for rapid imaging with relatively high signal-to-noise ratio of fluids such as blood. However, these sequences tend to produce bright fat signals which are in many cases undesirable, especially for instance when imaging the coronary arteries.
Various methods have been used to discriminate fat from water during SSFP imaging. Fluctuating equilibrium sequences, which adjust pulse phases in multiple images, can separate fat and water. Fat saturation has also been accomplished in SSFP with proper spin-preparation during periodic interruption with fat-saturation pulses. Interleaving fat-saturation pulses into the pulse sequence can disturb the steady-state equilibrium and affect water signals undesirably and, since not all parts of k-space are equally affected, can spread remaining fat signal to fat-free regions of the image. A method is needed for suppressing fat signals in steady-state free precession pulse sequences without the use of separate fat-saturation pulses.
In a first aspect, a method is provided for acquiring images with water and lipid separation using a magnetic resonance imaging (MRI) system. This method comprises applying a plurality of steady-state free precession (SSFP) radio-frequency (RF) excitation pulses having respective inter-pulse gaps between the pulses to discriminate lipids from water wherein the inter-pulse gaps alternate between a first width and a second width.
In a second aspect, a system for Magnetic Resonance Imaging (MRI) for acquiring images of an object is provided. The system comprises a magnetic field driver for driving a field gradient, a magnetic field controller for controlling the magnetic field, a transmitter for generating an radio frequency (RF) pulse to cause resonance and wherein the transmitter is adapted to generate a plurality of steady-state free precession (SSFP) radio-frequency excitation pulses having respective inter-pulse gaps between the pulses to discriminate lipids from water, the inter-pulse gaps alternating between a first width and a second width. The system further comprises a receiver for receiving and detecting magnetic resonance signals generated from the object and a processor for performing image reconstruction and for generating images for display.